Basically, 4chan is an image board website. Picture posting and text. Whereas Gaia is more of an avatar based website, with much more advanced posting syles/options.

Well, under the "Boards" section, you'll see various places to go to. Click a topic that interests you. Each "thread" is separated by a horizontal line. Directly under the horizontal line, you'll see some words, numbers, and "reply". Click reply, and you'll enter the entire topic.